Effect of Temperature, Acidity, etc. 419 



hydrogen ion concentration owing to the formation of sodium 

 acid sulphate. This hypothesis seems to be supported by the 

 fact that the precipitates from 2 per cent initial sulphuric acid 

 solutions were largely amorphous when sodium sulphate was 

 used, whereas when it was not used the products from 2 per 

 cent sulphuric acid were well crystallized, becoming amorphous 

 only when the acid concentration was reduced to less than 

 1 per cent. 



4. Time. — Since wurtzite is unstable below 1020°* it was 

 thought that a longer heating of the products with the solu- 

 tion out of which they crystallized would cause the wurtzite 

 to change into sphalerite. It was impossible to obtain con- 

 clusive evidence on this point owing to the fact that the time 

 could not ~be increased without a corresponding decrease in 

 acid concentration due to the reduction of the sulphuric acid 

 by the hydrogen sulphide, which had to be renewed continu- 

 ally to prevent solution of the zinc sulphide. The duration of 

 each experiment is given in the tables (X-XII) and it will be 

 seen that at 250° (Table X) (the reduction of the acid is slow 

 enough to allow a long heating at this temperature) there is 

 no increase in sphalerite in the products when they are heated 

 5 days instead of 3 days, provided the acid concentration does 

 not fall below a certain limit. The following experiment 

 shows that at 325° wurtzite is not chauged to sphalerite in one 

 day if the acid concentration is high but considerable change 

 occurs in one day if the acid concentration is low. 



F. Evidence that wurtzite is changed to sphalerite by heat- 

 ing ivith dilute acid. — A product which had been made from 

 a solution, the final acidity of which was 4*2 per cent, by heat- 

 ing for one day at 325°, consisted entirely of wurtzite. This 

 wurtzite was again put into a platinum tube with 3 per cent 

 sulphuric acid saturated with hydrogen sulphide and sealed 

 in a glass tube containing 0'5 g. ammonium thiocyanate and 

 0*66 g. sulphuric acid dissolved in 15 cc of water in the outside 

 tube. The air in the glass tube was replaced by hydrogen 

 sulphide before sealing. Except for the acid concentration 

 this tube was similar to the one in which the wurtzite had 

 been formed. After heating for one day microscopic examin- 

 ations showed that the wurtzite had well-formed new crystals 

 of sphalerite growing on it. The acid concentration had fallen 

 to 1*45 per cent owing to distillation and reduction. 



Additional evidence on this point is given in Table IX. 

 As has been stated (page 416), some zinc sulphide is pre- 

 cipitated from solutions having an initial acid concentration of 

 3 per cent or less before the tubes reach the temperature of 

 the furnace. When the furnaces were set for 300°, or higher 

 * Allen, Crenshaw and Merwin, loc. cit. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 227.— November, 1914. 

 29 



